Chapter 3: Plant Nutrient Requirements

3.5 Minor nutrients or trace elements

Although only required in small amounts, minor nutrients (micronutrients, or trace elements) are essential for plant growth. These nutrients often act as catalysts in chemical reactions. It is possible to have toxicities of trace elements, as well as deficiencies.

Many products in the market place extol the virtues of trace elements that are ‘absolutely needed’ by plants. Some companies use soil tests to determine whether trace elements are deficient in the soil. On the basis of many field and laboratory experiments and much experience, the Department of Primary Industries Victoria has found that soil tests are not a reliable method of detecting a trace element deficiency. Plant tissue tests (see Chapter 8.4) are far more reliable for assessing what was available for plant uptake, but even these are not always correct and must be taken at the appropriate times of the year to increase their accuracy and reliability. In addition, recommendations should be based on research conducted in Australian soils or on Australian plants, not on overseas data.

Some of the micronutrient deficiencies in plants can cause nutrient deficiencies in the animals that graze those plants. In some cases (for example, copper and manganese), these micronutrients are also essential for plant growth. In other cases (for example, selenium), they are not required by the plant. Thus, in many cases of animal nutrient deficiency, it may be better to treat the animal rather than to apply fertilisers to pastures to overcome the problem. It is therefore important to discuss trace element issues with your local veterinarian.

Though plant testing is the recommended method for testing for trace element disorders in plants, it is usually unreliable for trace element requirements for animal nutrition. Testing body fluids (blood, urine, saliva) and tissues (liver, bone) is often required to determine whether animals have a trace element disorder. Seeking veterinary advice in addition to, or instead of, plant tissue testing is recommended.

This manual only touches on the complex issue of trace elements and their deficiency and toxicity implications. Several high-quality publications containing colour photographs of deficiency and toxicity symptoms and descriptions are recommended for additional reading. See the References at the end of this chapter.

Some of the more common trace elements likely to be deficient in Australian soils are discussed in this section.

3.5.1 Molybdenum

3.5.1 Molybdenum

Molybdenum (Mo) is essential for the health of the rhizobia bacteria associated with the legume root nodules that are responsible for atmospheric nitrogen fixation.

Molybdenum is also directly involved in nitrogen metabolism and specifically implicated in the electron-transfer system (for example, nitrate reductase and enzyme nitrogenous reactions). Molybdenum is the least abundant of the trace elements in the soil and the least required by plants with the exception of nickel.

3.5.1.1 Molybdenum deficiency symptoms

Molybdenum deficiency symptoms may look similar to a nitrogen deficiency (see Section 3.4.1.1), and legumes will have green or grey to white nodules rather than the pinkish-coloured nodules of healthy plants.

Consequently, a lack of molybdenum will reduce the nitrogen-fixing ability and growth of legumes. In effect, molybdenum-deficient plants cannot properly metabolise nitrate nitrogen, even though their tissues may contain considerable amounts of nitrates.

Molybdenum is not sorbed by soil when soil pH (1:5 CaCl2) is greater than 5. Sorption occurs when soil pH is below 5, with sorption increasing as soil pH increasingly falls below 5. Therefore, deficiencies are more likely in acid soils. The application of lime increases soil pH improving the effectiveness of naturally occurring molybdenum present in soil, thereby increasing availability for plant uptake. However, peat soils, which are usually acidic, should not require additional molybdenum, as these soils usually have high levels of molybdenum held within the organic matter. The application of lime to the peaty soils in the Koo-wee-rup area of southern Victoria has been sufficient to rapidly increase the availability of molybdenum and sometimes induce a copper deficiency in livestock.

Correcting a molybdenum deficiency can be by the application of a fertiliser to the soil or in water. Fertiliser products such as superphosphate can be used that have molybdenum added. However if lime is being applied the increase in soil pH may increase the solubility of soil molybdenum preventing any further application.

Applications of 50 to 200 g Mo/ha every 3 to 10 years are required for correcting pastures deficiencies and 50 to 60 g Mo/ha every 5 to 7 years are required on responsive soils for field crops. However, a plant tissue test (see Chapter 8.4) may show that Mo is not required at the ‘due time’.

3.5.1.2 Animal health implications

A complex relationship exists between copper, molybdenum, and the sulphates in animal nutrition.

Copper and molybdenum are mutually antagonistic in plant uptake; that is, if one is applied, uptake of the other may be reduced. Conversely, an oversupply of copper can induce a molybdenum deficiency in animals, particularly on lighter-textured soils or when animals are stressed.

Molybdenum toxicity (molybdenosis in ruminants) is not thought to be significant in plants, but excessive molybdenum levels in plants (forage crops) or high rates of molybdenum applications in fertilisers can sometimes induce copper deficiency in livestock. There are complex reactions in the animal’s rumen involving molybdenum, sulphur and copper; generally forming a complex product with copper becoming unavailable to the animal. Often these conditions can be subclinical and difficult to diagnose. Therefore copper:molybdenum ratios of animal diets should be monitored and maintained with suitable ranges (5:1 to 10:1).

Sulphate can also sometimes restrict molybdenum uptake by plant roots when the two nutrients are applied together. A high molybdenum content in plants can be dramatically lowered by the addition of sulphates. To avoid copper deficiency in grazing animals, it is generally recommended that copper should be applied when molybdenum is being applied in a fertiliser. However, if plant levels of copper (as determined by a plant tissue analysis; see Chapter 8.4) are at satisfactory levels or higher, then molybdenum can be applied without copper.

In addition, molybdenum should not be applied to pastures limed within the past 12 months, as the combination of applied molybdenum plus the molybdenum released from the soil by the lime may raise the molybdenum levels enough to cause copper deficiencies in livestock.

3.5.2 Copper

3.5.2 Copper

Copper (Cu) is required for the formation of enzymes for chlorophyll production, nutrient processing and the plant’s exchange of water and oxygen for carbon dioxide. It is also required for seed setting of legumes. Plant responses (in other words, additional growth) due to copper are rare. Like most trace elements excessive quantities of copper can interfere with the uptake of other trace elements like iron; therefore producing iron deficiency symptoms.

3.5.2.1 Copper deficiency symptoms

Copper deficiency symptoms are not very specific in plants, although ‘dieback’ is common, showing up first in the young growth. Copper deficiencies commonly occur in highly leached acid sands (such as coastal sandy and sandy loam soils), in loams from sandstone, in peat soils, and in highly calcareous alkaline soils. Heavy clay type soils are least likely to be copper deficient.

Copper deficiencies can be correctly by either soil or foliar applications. Copper can be applied as an impurity in some common phosphorus fertilisers, but generally copper is broadcast onto pastures annually using superphosphate with copper as an additive. Broadcasting copper fertiliser is less effective than incorporation as a band into the soil prior to planting, due to its immobility. Because copper is immobile it remains in the soil for plant uptake.

Foliar applications of copper sulphate (bluestone) can be effective in correcting copper deficiency in an existing crop or pasture, however caution should be exercised as the mixture can burn under adverse environmental conditions (e.g. hot dry windy conditions).

Rates of 1.5 to 2 kg Cu/ha applied as fertiliser every 3 to 6 years are required for deficient soils.

3.5.2.2 Animal health implications

Problems with copper are more commonly associated with animal deficiencies than with plant deficiencies. Because animals have a higher copper requirement than plants do, animals may become deficient at copper levels that are sufficient for normal plant growth. Imbalances with excessively high trace elements in the soil (e.g. iron, molybdenum – see Section 3.5.1.2) can induce copper deficiencies in animals with the animal showing typical unthrifty symptoms increasing as body reserves are depleted.

Copper deficiency symptoms are also more obvious in livestock than in plants. The symptoms appear as hair or coat abnormalities (red-coated animals tend to be pale-red or orange), retarded growth and skeletal defects, infertility, and diarrhoea.

It may be necessary to treat the animals directly, particularly if copper deficiency symptoms are evident in livestock.

3.5.3 Zinc

3.5.3 Zinc

Zinc (Zn) is associated with the formation of chlorophyll and of several enzyme systems required for protein synthesis. It also has a regulatory role in the intake and efficient use of water by plants.

3.5.3.1 Zinc deficiency symptoms

Zinc remains in the older green leaves as it is poorly mobile, so as the deficiency increases symptoms will appear in the middle to younger leaves, deficiency symptoms include:

Small bronze spots on older leaves of legumes; as spots enlarge, leaves develop a mottled appearance.

Branching of small, dark green, distorted leaves in the centre of legume plants (called the ‘little leaf syndrome’ and noted at Yanakie, South Gippsland).

Typically, zinc deficiency is associated with leached acidic sandy soils, alkaline soils with considerable calcium carbonate content, and soils with high organic matter. Deficiencies may be temporarily induced by cold, wet weather and have been noted to disappear with the onset of warmer weather.

Zinc deficiency affects millions of hectares throughout Australia. It is associated with many crops and pastures grown across a wide range of soil types, including coastal pastures along eastern and Western Australia and large areas of alkaline cracking soils throughout the dairying regions of all states within Australia. Deficiencies are uncommon in pastures in southern Victoria except on the alkaline coastal soils.

Zinc availability is related to pH; and in the north-west and Goulburn Valley areas, zinc availability is often low on heavily cut laser levelled paddocks after landforming, particularly if they are planted to maize and other fodder crops. In this situation, alkaline subsoils become exposed.

Zinc deficiencies can be successfully corrected by either soil or foliar applications. As zinc is immobile in the soil, placement of zinc close to the developing root system is important. Zinc can be applied to the soil in smaller amounts as a component of some pre-plant fertilisers (e.g. Granulock Z which contains 1% Zn), or in higher concentrations lasting for many years (e.g. zinc sulphate monohydrate which contains 35% Zn). Zinc can also be applied to existing pastures using superphosphate with zinc added. Foliar applications of a zinc sulphate or a chelated form can be applied to an existing crop or pasture to correct an existing deficiency or soil conditions that have reduced the availability of zinc.

3.5.4 Manganese

3.5.4 Manganese

Manganese (Mn) has several plant-growth functions. It is closely associated with iron, copper and zinc as a catalyst in plant-growth processes; is essential for rapid germination; and plays a role in enzyme systems in seed and new tissues.

3.5.4.1 Manganese deficiency symptoms

Manganese deficiency symptoms include:

Yellowing between the veins of young leaves due to immobility in plants.

Eventually spots of dead tissue may drop out, leaving a ‘ragged’ leaf.

Stunting of growth.

Reduced flower formation.

The main factors affecting manganese availability are soil pH and seasonal variability.

The more alkaline the soil, the more likely deficiencies will occur. Conversely, very strongly acidic soils can accumulate toxic levels of manganese – see Section 3.5.4.2. Occasionally, a manganese deficiency can be induced by excessive liming on these acid soils.

Manganese deficiency is more often associated with coastal calcareous soils and deficiencies are more likely to occur in highly alkaline soils with high organic matter.

Due to the seasonal availability of manganese, symptoms may be more prominent in the cooler wetter months and disappear during the warmer months – see Figure 3.12

There is no evidence in Australia of manganese deficiency affecting pasture growth; however it is more common on alkaline cropping soils during the cooler months. Manganese deficiency in pastures can be treated by applying manganese sulphate.

3.5.4.2 Manganese toxicity symptoms

Manganese toxicity symptoms include:

In sensitive plants (Lucerne and clover), symptoms of toxicity appear in late autumn, initially as a light brown discolouration of the leaf margins, which later become reddish. Waterlogging or root rot can produce similar symptoms, so a plant tissue analysis may be necessary to determine the true problem.

The plant may die in cases of severe toxicity.

Although rare, manganese toxicity can occur during the warmer months in high rainfall, acid soils (<4.3 pH CaCl2) inherently high in manganese. Figure 3.12 shows the variation in manganese availability with seasonal conditions.

Manganese toxicity has been found in the northern tropical regions of Australia where high rates of acidifying fertiliser have been used on already acid soils. In these regions, there can be a five-fold increase in manganese availability during the warmer wetter months, and possible toxicity.

Manganese toxicity is also more likely when grazing lupins as they can accumulate high concentrations of manganese. Soil compaction and waterlogging (both of which result in inadequate soil aeration) can produce manganese toxicity in plants; especially in more susceptible crops like canola, lucerne, phalaris and annual medics. – See Table 3.3.

Manganese toxicity can be reduced by working lime into the soil to a depth of 100 to 150 mm and by correcting waterlogging and soil compaction.

3.5.4.3 Animal health implications

Livestock are susceptible to both manganese toxicities and manganese deficiencies.

A lack of manganese is commonly associated with infertility in cows and impaired growth and bone development. There have been no confirmed cases of manganese deficiency in grazing animals in Australia.

Deficiencies in livestock can be corrected with manganese supplements. Check manganese levels via a plant tissue analysis of mixed herbage if concerned about manganese deficiency in livestock – see Chapter 7.3.2)

3.5.5 Iron

3.5.5 Iron

Iron (Fe) is associated with the production of chlorophyll and helps to carry oxygen around the plant cells. Iron is also involved in reactions that convert nitrates to ammonia in the plant.

3.5.5.1 Iron deficiency symptoms

Iron deficiency symptoms include:

Chlorosis (yellowing) between the leaf veins of the youngest leaves.

Tips and margins of leaves remain green for the longest time.

Affected leaves curve upwards.

Stunting and abnormal growth.

Iron is very immobile in the plant. Thus, deficiency symptoms affect the youngest leaves first.

Deficiencies usually occur on high-pHcalcareous soils, waterlogged soils or in soils that have been heavily limed. The correction of iron deficiencies is generally through the application of foliar iron fertilisers; iron sulphate or iron chelate. If iron is applied to the soil it can be converted to an unavailable form, particularly when applied as iron sulphate.

3.5.6 Boron

3.5.6 Boron

Boron (B) is mainly involved in the movement of sugars throughout the plant and in seed production in legumes. It is also an important nutrient in the metabolism of nitrogen, carbohydrates, and hormones and is involved in the uptake and efficient use of calcium in the plant.

Boron may induce both toxicities and deficiencies in Australia.

3.5.6.1 Boron deficiency symptoms

Boron deficiency symptoms include:

Distorted and chlorotic leaves with darker pigmentation along the leaf margins.

Red and yellow discolouration, particularly in sub clover.

Poor growth.

Low seedset.

Deficiencies often tend to disappear after rainfall since plant roots may be unable to access soil boron in dry soils. Lucerne is the main crop in which boron deficiency has been identified in Australia

Boron deficiencies may occur in humid regions, in highly leached acid sands, in organic (peaty) soils, and in calcareous (alkaline) soils and becomes less available in poorly drained soils.

Occasionally, liming may heighten a boron deficiency. Boron deficiency can be induced in turnip fodder crops by lime application, usually at 3.5 t/ha or higher during seedbed preparation. Boron starts to be sorbed by soil at pH (1:5 CaCl2) values greater than 7-8, with sorption thereafter increasing as pH values increase.

If plant tissue analysis (see Chapter 8.4) indicates a deficiency, then apply boron with a fertiliser application and retest in 2 to 3 years. Seek expert advice to determine the appropriate boron types and application rates. A problem with boron is that amounts required to overcome a deficiency and amounts causing toxicity for plant production are relatively close; so avoid applying too much fertiliser boron. Once induced, toxicity is difficult to ameliorate.

3.5.7 Chlorine

3.5.7 Chlorine

Chlorine (Cl) is thought to stimulate carbohydratemetabolism, some plant enzymes, chlorophyll production, and the water-holding capacity of plant tissues. Chlorine seems to be more important for animals than for plants. Deficiencies of chlorine seldom occur as the chloride ion is continually replenished via rain water, the amount increasing with rainfall quantity and closeness to the sea.

3.5.8 Nickel

3.5.8 Nickel

Nickel (Ni) is a naturally occurring element found in soil, water, air and biological materials. Its availability in soils is very pH dependant, with the nutrient becoming soluble and therefore plant available when less than pH 6.5 (water). Because nickel is required by plants in such small concentrations (0.1 to 5 mg Ni/kg dry weight) detection of plant deficiencies is difficult in the field.

Nickel is a very mobile nutrient within plants with large proportions being rapidly translocated to seeds from shoots. Nickel has the following roles in plants:

As a component of the enzyme urease, it is involved in nitrogen metabolism

Involved in nitrogen translocation within plants

Involved in bacterial enzymes; including nitrogen fixation.

Involved in the seed germination and vigour

Influences plant disease resistance

Nickel is unlikely to become deficient in soils due to its small plant requirement and natural presence throughout most soils. A nickel deficiency, however, could be exacerbated by excessively high applications of other nutrients (zinc, copper, manganese and iron); root damage by nematodes; and cold wet conditions. Nickel is present in cow’s milk at 0.1 mg Ni/L and in normal circumstances would not be required as a separate dietary component. However, if it is not present in the animal in sufficient quantities, the main adverse effect is a reduced feed intake and reduced growth.

Definitioner

the most common form of phosphate fertiliser used in Australia: Ca(H2PO4)2.

sorbed (sorb)

fix to the soil

sorption (sorb)

fix to the soil

soluble

the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution.

soil types

the classification of a soil due to its formation which is influenced by parent material, climate, topography, organic activity and age.

soil tests (soil test)

the analysis of soil sample to determine nutrient content, composition and other characteristics, such as acidity or pH level.

seedset

fertilised ovules that mature into viable seeds.

rhizobia

soil bacteria that fix nitrogen after becoming established inside root nodules of legumes

plant tissue tests

an assessment of the nutrient content of a plant by testing a sample of tissue from that plant.

pH

a measure of soil acidity or alkalinity on a scale of 0 (extremely acidic) to 14 (extremely alkaline) with a pH of 7 being neutral. It gives an indication of the availability of plant nutrients.

peaty soils (Peat soil)

soil with an accumulation of partially decayed vegetation

peat

a heterogeneous mixture of more or less decomposed plant (humus) material that has accumulated in a water-saturated environment and in the absence of oxygen.

organic matter

anything that is either living or the remains of a living thing; organic matter in the soil refers to the remains of living things, namely, decayed animal and plant matter.

Organic

has come from a once-living organism, is capable of decay or the product of decay, or is composed of organic compound.

nitrogen fixation

any process (either chemical or bacterial) that describes the transformation of N2 gas from the atmosphere to nitrogenous compounds.

nematodes

round worms.

mobile

(of a nutrient in the plant) moves to the actively growing tissue, such as root tips and growing points in the tops of plants; (of a nutrient in the soil) readily leached.

micronutrients

nutrients essential to plants but only required in small amounts.

micronutrient (micronutrients)

nutrients essential to plants but only required in small amounts.

trace element (micronutrients)

nutrients essential to plants but only required in small amounts.

metabolism

the chemical processes that occur within a living organism in order to maintain life

metabolism

the chemical processes that occur within a living organism in order to maintain life

lime

a naturally occurring calcareous material used to raise the pH of an acidic soil.

legume

a member of the Leguminoseae family. Characterised by their pea-shaped flowers. Legumes have a symbiotic association with the nitrogen-fixing species of the bacterial genus <em> Rhizobium</em>. (See symbiotic fixation.)

legumes (legume)

a member of the Leguminoseae family. Characterised by their pea-shaped flowers. Legumes have a symbiotic association with the nitrogen-fixing species of the bacterial genus <em> Rhizobium</em>. (See symbiotic fixation.)

leaf margins (leaf margin)

the border or edge of a leaf

ion

an atom in water that is either positively or negatively charged.

fixation

the rendering insoluble or making unavailable, by elements or organisms within a soil, of a plant nutrient: Phosphorous is fixated by iron and aluminium oxides; nitrogen is fixated by bacteria in the nodules on legumes for use by the host plant.

fertiliser

any nutrient or substance that, when added to the soil, promotes plant growth. This definition includes both inorganic and organic fertilisers and also soil conditioners, such as lime and gypsum, which may promote plant growth by increasing the availability of nutrients that are already in the soil or by changing the soil’s physical structure.

fertilisers (fertiliser)

any nutrient or substance that, when added to the soil, promotes plant growth. This definition includes both inorganic and organic fertilisers and also soil conditioners, such as lime and gypsum, which may promote plant growth by increasing the availability of nutrients that are already in the soil or by changing the soil’s physical structure.

enzymes

large biological molecules responsible for the thousands of chemical interconversions that sustain life.

enzyme (enzymes)

large biological molecules responsible for the thousands of chemical interconversions that sustain life.

element

simple substance that can not be broken down into simpler substances by normal chemical means.

elements (element)

simple substance that can not be broken down into simpler substances by normal chemical means.

dieback

a condition of plants that starts and the tips of the shoots and works downwards, causing progressive lifelessness.

compaction

the process whereby soil density is increased as a result of tillage, stock trampling and/or vehicle traffic. Compaction can lead to lower soil permeability. Deep ripping and conservation tillage can alleviate the condition.

chlorosis

the yellowing or whitening of normally green plant tissue because of a decreased amount of chlorophyll, often as a result of disease or nutrient deficiency.

chlorotic (chlorosis)

the yellowing or whitening of normally green plant tissue because of a decreased amount of chlorophyll, often as a result of disease or nutrient deficiency.

catalyst

a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change

catalysts (catalyst)

a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change

carbonate

a salt of the anion CO32−, typically formed by reaction of carbon dioxide with bases

calcium carbonate

a crystalline compound (CaCO3) occurring in nature as the mineral calcite. It is the neutralising component of lime.

calcareous

a soil that has sufficient calcium carbonate to cause effervescence when a few drops of hydrochloric acid are applied to it.

broadcast

process of spreading fertiliser products onto the soil surface.

aeration

refers to the process of using mechanised equipment to loosen the soil.